"BigTom" schreef in bericht ...
Jerry G. wrote:
This is not something that can be done over an email. It is the skill and
experience of using the proper tools and knowledge to be able to trace
through the operation of the supply, and to be able to check and or try
out
the various suspect components.

Many times in these supplies the electrolytic caps give problems. You can
use an ESR meter to test them.


ive checked esr on all the caps and theyre ok.
ive got a another working monitor to test voltages/resistances etc.
ive read the sci.electronics.repair.of.smps.
are there any other good web sites where i can learn more about smps.



From sci you should know the SMPS is starting but switches off before it is
really up and running. From the datasheet of the 3842 you should at least
have a general idea of the workings of that chip, especially from the
example circuit. Now you have two main causes that makes the regulator
switch off:

- Overcurrent. Somehow the voltage across the current sense resistor becomes
too high. Maybe a short in the transformer but an overload or short on the
secondary side is also possible. I ever had a current sense resistor that
has got to high a value.

- Power failure. These regulators start using a start circuit that provides
just enough current just to start. When the circuit comes up it requires
more current which usually is provided by the transformer. (Just look at the
example on the datasheet .) This piece of power supply may fail itself but
the failures mentioned above may also cause this problem.

Failing transformers can be found by using a ringer. Secundairy overloads or
shorts can often be found using an Ohmmeter. Pay special attentiion to the
secondary rectifiers and their smoothing capacitors. Keep in mind that parts
that look all right when measured this way nevertheless may fail under load.
Also keep in mind that the cold filament has a low resistance. If this all
does not give a clue you can try to find the failing secondary part by
temporary removing the rectifier. (Which is not riskless but your last
chanche.)

petrus bitbyter